The lethal and sub-lethal toxicity of LAS congeners to the mollusc gastropod Hydrobia ulvae were assessed in spiked sediment bioassays. This complements the little knowledge available to date on mixture effects in the sediment compartment. The LAS homologues joint effect was concentration additive (?TUi = 0.8–1). As opposed to the 10-d LC10 based on the sediment associated LAS concentration (91–330 mg/kg) which was independent of the homologue chain length, the LC10 based on the dissolved LAS fraction (0.804–0.068 mg/L) decreased as the homologue chain length increased from 10 to 13 carbons. The quantitative structure-activity relationship (QSAR) derived from these data was log (1/LC10 (mol/L)) = 0.64 log Kow + 4.40 (n = 5; r2 = 0.76; s = 0.24). It showed an apparent higher toxicity compared to the typical QSAR for polar narcosis in water-only systems probably due to the simultaneous exposure of the snail to LAS through the dissolved and the sediment associated fractions. The egestion rate of the surviving snails recovered after few days' exposure (1-d NOEC: 40–107 mg/kg, 9-d NOEC: 65–190 mg/kg) which suggests that the organisms were able to acclimate to LAS during the exposure.